MQTTv5 client cannot connect to broker that only supports max. MQTTv3.1.1

- [x ] Bug exists Release Version 1.2.5 ( Master Branch) MQTTv5
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.6-SNAPSHOT (Develop Branch): not tested

**Test with (very old) mosquitto version 1.5.5 (MQTTv5 client)**
Debug-Messages from mosquitto:
New connection from ....
Invalid protocol version 5 in CONNECT from ....
Sending CONNACK to ...
Socket error on client , disconnecting.

**Test with (very old) mosquitto version 1.5.5 (MQTTv3 client)**
no error
Debug-Messages from mosquitto:
New connection from...
New client connected from ...

There is no way to set the MQTT version as in the MQTTv3 client (setMqttVersion), because the method is missing. Is no automatic fallback to V3.1.1 implemented?

Reproduce the connection against mosquitto 1.5.5 using the MQTTv5 client and compare it with the MQTTv3 client's setMqttVersion behavior. Trace the client connection and protocol-version handling; done means the client can connect to a broker supporting only MQTT 3.1.1, either through version selection or the requested fallback behavior.

Written by the indexing model from the issue text.